reading serial port

Using Arduino Programming Questions
21

OP, it looks like you're confusing "symbol" (aka bits) with characters.
That is not the case.
The number of information items, or symbols, is 115200 per second,or 115200 baud.
That is the information rate, but bits have to be combined into characters, and the characters have to be separated.

22

If you know the message len, or if the message length never change in all the communications, there is a simple way to realize the serial communication.
I think I've solve my issue with this code

long interval = 3000;
long previous = 0;
char string[300];
int i=0;
int message_len = 230;

void setup() {
  int j;
  Serial.begin(115200);
  for(j=0; j<300; j++) {string[j] = '\0';}
}

void loop() {
  unsigned long start = millis();
  byte p;
  char c;
  int j;
  
   if (Serial.available()>0) {      
      c=Serial.read();
      string[i] = c;
      string[i+1] = '\0'
      i++;
   }
   if ( i == message_len) {
     Serial.print("string: ");
     Serial.println(string);
     previous = start;
     i=0;
  }
 if (start-previous > interval) { 
  Serial.println("Alive");
 }

}

Obviously this code is not flexible, but works well in many situations.
Thank you all,